A wide variety of devices are available today that employ cellular technology for wireless communications purposes, and an array of cellular systems exist to provide the supporting cellular infrastructure for these devices. Numerous underlying radio access technology (RAT) standards exist that may be used to provide this cellular infrastructure, such as GSM, 3GPP and 3G beyond. The ability to perform a cell search to find the supporting cellular infrastructure is a fundamental requirement in any cellular device. Moreover, the ability to quickly perform a cell search is highly desirable, as this provides for cellular handoffs with fewer dropped data links, and faster connection times after power-on or out-of-service conditions.
Increasingly, consumers are demanding that functions that were once provided by two or more devices be consolidated into a single device. As these functions may conceivably employ different RATs, a single device may have to be able to perform cell searching across a multiplicity of RATS. Additionally, the RATs themselves undergo generational evolution to provide enhanced functionality. Backward compatibility between these generations may be highly desirable in some devices, such as cellular telephones that support older GSM, newer 3GPP technologies, and cutting-edge 3GBeyond.
With reference to FIG. 1, a cellular device 10, such as a PDA or a mobile phone, includes an upper layer 20 in communications with a lower, physical layer 30. The upper layer 20 is typically implemented in software, while the lower layer 30 may be a combination of both hardware and software. The physical layer 30 includes an antenna 32, a low noise amplifier 34 and a radio-frequency (RF) pre-processor 36, which together form a receiver 38. The LNA 34 is an analog device that amplifies the relatively weak RF signals provided by the antenna 34, and feeds an amplified signal into the RF pre-processor 36. RF pre-processor 36 typically down-converts the amplified signal into a baseband or intermediate frequency (IF) signal, and then utilizes an analog-to-digital converter (ADC) to convert the baseband or IF signal into a corresponding stream of digital samples. The type of conversion employed by the ADC will depend upon the type of modulation used by the RAT, such as binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), quadrature amplitude modulation (QAM), differential phase-shift keying (DPSK), etc. This digital stream is then used by a cell search engine 40.
The cell search engine 40 within the lower layer 30 typically includes a digital signal processor (DSP) 46 connected to both a non-volatile memory holding program code 44, and a working buffer 46. The working buffer 46 is generally some type of fast memory, such as RAM or internal registers, and is used by the DSP 42 to hold and manipulate digital data provided by the RF pre-processor 36. The program code 44 is executable by the DSP 42, and contains algorithms necessary to support cell searching as determined by the RAT standards, such as digital demodulation, descrambling, channel decoding, slot synchronization, frame boundary detection, Gold code detection, or the like. Alternatively, the cell search engine 40 may be implemented with dedicated specific digital signal processing building blocks that may be controllable or even programmable, as known in the art.
A cell search begins with the upper layer 20 sending a frequency scan request for a frequency to the lower layer 30 to determine if there are any cellular stations nearby operating on the frequency. The lower layer 30 scans the frequency and generates frequency scan results 46a, which are passed up to the upper layer 20. Based upon the frequency scan results 46a, the upper layer 20 may request from the lower layer 30 a cell search procedure within the frequency, or a scan of a new frequency. To perform the cell search procedure, the upper layer 20 instructs the lower layer 30 to perform a cell search on a particular frequency. The cell search engine 40 iterates through each RAT supported by the device 10, performing a cell search procedure as determined by each RAT protocol, which can be quite complex, and passes cell search results 46b up to the upper layer 20 for that frequency. This process is iterated over each frequency of interest, which are typically those frequencies having the highest power. Based upon the cell search results 46b, the upper layer 20 may determine that no suitable cell exists within the searched frequency, and so initiates searching in a new frequency; or, the upper layer 20 may decide that a suitable cell has been found, and may then attempt to connect to the cell.
If multiple RATs are to be supported over several frequency bands, the serial nature of the cell searching process can lead to quite lengthy cell search times, and to large amounts of communications between the upper layer 20 and lower layer 30. It would therefore be desirable to provide a cell searching method, and related device, that provides for rapid cell searching across multiple radio access technologies and frequency bands.